We have been on Princess, Celebrity, Carnival, and RCI, all with our teens. They are both girls (15 and 12) and we require them to dress formally on formal nights. This is easier for my high schooler because she has formal dresses from prom etc. and she just uses them. The 12 year old just gets a couple of new dresses each time we cruise because she still grows out of them between cruises! The problem for us arises with the 15 year old and the casual nights. We ALWAYS want them with us in the dining room because sometimes it is the only time we really get to talk all day
However, my 15 year old especially wears jeans, shorts, or jean skirts almost exclusively at home. She does have one pair of dress black pants that she can wear. However, I agree with the person who posted earlier talking about the difference in types of jeans. My girls both have the "Abercrombie" jeans with holes that we paid for, but those would never be appropriate in the dining room. However, for a teen to wear a pair of very nice Guess type jeans with a fancy blouse and heels will make them look more dressed up than many of the others in the dining room. When we were on Princess last year, that is what I saw from the majority of the girls. I am all for the formality of cruising and dressing up, it is one of the things I like most. However, going out and buying ugly clothes my child will never wear again just to follow the letter of a rule no one is enforcing anyway seems silly. I prefer to go with the intent, which is to dress nicely and act as though you are at a nice restaurant in your hometown.